U.S. patent application number 12/375138 was filed with the patent office on 2009-12-10 for optical disc medium and optical disc device.
This patent application is currently assigned to Mitsubishi Electric Corporation. Invention is credited to Kenya Nakai, Masahisa Shinoda, Nobuo Takeshita.
Application Number | 20090303850 12/375138 |
Document ID | / |
Family ID | 38981300 |
Filed Date | 2009-12-10 |
United States Patent
Application |
20090303850 |
Kind Code |
A1 |
Nakai; Kenya ; et
al. |
December 10, 2009 |
OPTICAL DISC MEDIUM AND OPTICAL DISC DEVICE
Abstract
When using an optical disc medium that includes pit trains
having their widths narrower than a diffraction limit, it is
difficult to detect a tracking error signal and take a
tracking-servo control while increasing pit density in a direction
orthogonal to a pit-train extension direction. Information pit
trains are arranged spirally or concentrically and formed in a
structure in which their depths are changed periodically at a pitch
radially along the optical disc medium, so that the tracking error
signal can be obtained by push-pull detection of diffraction light
from the structure.
Inventors: |
Nakai; Kenya; (Tokyo,
JP) ; Shinoda; Masahisa; (Tokyo, JP) ;
Takeshita; Nobuo; (Tokyo, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
Mitsubishi Electric
Corporation
Tokyo
JP
|
Family ID: |
38981300 |
Appl. No.: |
12/375138 |
Filed: |
May 22, 2007 |
PCT Filed: |
May 22, 2007 |
PCT NO: |
PCT/JP2007/060402 |
371 Date: |
March 26, 2009 |
Current U.S.
Class: |
369/53.15 ;
369/275.4; G9B/20.046; G9B/27.027 |
Current CPC
Class: |
G11B 7/24 20130101; G11B
2007/24316 20130101; G11B 2007/24308 20130101; G11B 7/0941
20130101; G11B 7/24085 20130101; G11B 7/0901 20130101; G11B
2007/2431 20130101; G11B 2007/24314 20130101 |
Class at
Publication: |
369/53.15 ;
369/275.4; G9B/20.046; G9B/27.027 |
International
Class: |
G11B 20/18 20060101
G11B020/18; G11B 7/24 20060101 G11B007/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 27, 2006 |
JP |
2006-204037 |
Claims
1-20. (canceled)
21. An optical disc medium that includes information pit trains
structured with information pits arranged circularly or spirally,
wherein the depths of the information pits are changed periodically
at a pitch radially along the optical disc medium, and the width of
each of the information pits is less than or equal to a
light-condensing spot's diffraction limit .lamda./(4.times.NA)
determined by a numerical aperture NA of an objective lens and a
wavelength .lamda. in use, as specifications of an optical disc
device records and/or plays back using the disc medium.
22. An optical disc medium that includes information pit trains
structured with information pits arranged circularly or spirally,
wherein the depths of the information pits are changed periodically
at a pitch radially along the optical disc medium, and the
information pits include those having a length in the circular or
spiral arrangement direction less than or equal to a
light-condensing spot's diffraction limit .lamda./(4.times.NA)
determined by a numerical aperture NA of an objective lens and a
wavelength .lamda. in use, as specifications of an optical disc
device records and/or plays back using the disc medium.
23. The optical disc medium according to claim 21, wherein the
depths of the information pits are changed periodically at a pitch
greater than or equal to a light-condensing spot's diffraction
limit as .lamda./(2.times.NA) determined by a numerical aperture NA
of an objective lens and a wavelength .lamda. in use, as
specifications of an optical disc device that records and/or plays
back using the disc medium.
24. The optical disc medium according to claim 23, wherein the
pitch at which the depths of the information pits are periodically
changed is one of approximately 0.32 .mu.m, approximately 0.74
.mu.m, and approximately 1.6 .mu.m.
25. The optical disc medium according to claim 21, wherein an
optical disc substrate having the information pit structure is
overlaid, in at least one layer, with a light-absorbing material
whose optical transmittance characteristic is nonlinear with
respect to light intensity.
26. The optical disc medium according to claim 25, wherein the
light-absorbing material includes Sb or Te.
27. The optical disc medium according to claim 25, wherein the
light-absorbing material includes at least one metal oxide of ZnO,
SnO.sub.2, TiO.sub.2, and Ta.sub.2O.sub.3.
28. An optical disc device that records onto and/or plays back from
the optical disc medium according to claim 25, wherein light
intensity at a part of a light-condensing spot where the objective
lens of the optical disc device focuses light on the optical disc
medium is greater than or equal to a threshold value that makes the
light-absorbing material produce super-resolution effect.
29. An optical disc medium that includes information pit trains
structured with information pits arranged circularly or spirally,
wherein the depths of the information pits are changed periodically
at a pitch radially along the optical disc medium, and a track
group includes a plurality of the information pit trains.
30. The optical disc medium according to claim 29, wherein the
track group has at least every one round of the medium a track
switching section in which a header is provided including at least
address information and/or tracking control information.
31. The optical disc medium according to claim 30, wherein the
information pits have a first pit depth and a second pit depth and
the depths of the information pits are changed periodically at a
pitch A, and at the track switching section each of the information
pit trains is connected to that shifted by half of the pitch A.
32. The optical disc medium according to claim 30, wherein the
information pits have a first pit depth and a second pit depth, the
depths of the information pits are changed periodically at a pitch
A, and at the track switching section, each of the information pit
trains is connected to that next to the each train.
33. The optical disc medium according to claim 21, wherein a light
phase difference caused by the change of the information pit depth
is between approximately 2.lamda./16 and approximately
6.lamda./16.
34. An optical disc device that records onto and/or plays back from
an optical disc medium according to claim 21.
35. An optical disc device that records onto and/or plays back from
an optical disc medium according to claim 22.
36. An optical disc device that records onto and/or plays back from
an optical disc medium according to claim 29, wherein the disc
device is provided with light receiving faces, for light reflected
by the optical disc medium, divided by a dividing line parallel to
a direction in which the information pits are at least circularly
or spirally arranged, and the disc device takes tracking-position
control of a light-condensing spot using a push-pull signal
obtained through a differential calculation of electrical signals
outputted from the light receiving faces.
37. The optical disc device according to claim 36, wherein by
setting a polarity and an offset value for a tracking error signal
corresponding to a position of the information pit train,
tracking-servo operation points are switched over so that the
light-condensing spot is controlled to be positioned to a desired
information pit train.
Description
TECHNICAL FIELD
[0001] The present invention relates to optical disc media and
optical disc devices, especially to an optical disc media capable
of enhancing data density and an optical disc device using the
media.
BACKGROUND ART
[0002] Until now, increasing of the capacity of various optical
discs has been realized in such a way that the size of
light-condensing spot is reduced on a focal plane, by downsizing
information pits formed on the disc tracks, as well as by adopting,
for recording and playing back use, a laser beam with a shorter
wavelength and an objective lens with a larger numerical
aperture.
[0003] For example, in a CD (Compact Disc) system, the disc
substrate serving as a light transmitting layer (a transparent
cover layer and a spacer layer provided on an information recording
layer, which are also called as a transparent substrate) is
approximately 1.2 mm in thickness, the laser beam wavelength is
approximately 780 nm, the numerical aperture (NA) of the objective
lens is 0.45, and the recording capacity of the CD is 650 MB.
Meanwhile, in a DVD (digital versatile disc), the disc substrate
serving as its light transmitting layer is approximately 0.6 mm in
thickness, the laser beam wavelength is approximately 650 nm, the
NA is 0.6, and the recording capacity of the disc is 4.7 GB. In
DVD, two substrates whose thickness is, for example, 0.6 mm are
bond together to be used as a 1.2 mm-thickness disc.
[0004] In a higher-density BD (Blu-ray Disc), a large memory
capacity over 23 GB has been realized using an optical disc with a
thinner light transmitting layer of 0.1 mm, by adopting a laser
beam wavelength of approximately 405 nm and a NA of 0.85.
[0005] In addition to the discs described above, in an HDDVD (high
definition digital versatile disc), a large memory capacity over 18
GB has been realized by using an optical disc substrate which
serves as light transmitting layer and whose thickness is 0.6 mm as
thick as the DVD, and by adopting a laser beam wavelength of
approximately 405 nm and a NA of 0.65. Moreover, optical disc
technologies are expected to further increase the density higher
than those described above.
[0006] One of the technologies for increasing data density on
optical discs is a super-resolution technology by which recording
marks or information pits are formed on an optical disc in a size
smaller than the diffraction limit and by which data is played back
from the marks or the pits formed on the optical disc.
[0007] Generally, in a playback method in which a light beam with a
wavelength .lamda. is used and focused with a numerical aperture NA
to produce a light spot, it is impossible to read data when a mark
pitch (or a pitch for information pit trains) is less than or equal
to .lamda./(2.times.NA); therefore, the pitch is referred to as the
diffraction limit. Assuming that a recording mark portion (an
information pit portion) has the same length as a spacer portion
within one pitch, the diffraction limit of the recording mark
length (or the information pit length) is given as
.lamda./(4.times.NA).
[0008] A super-resolution technology applied for recording and
playback includes, for example, a technique in which nonlinear
absorbing material that changes the refractive index or
transmittance according to the light intensity is used for the
optical disc medium to record therein marks or information pits
smaller than the diffraction limit using light with locally
intensified distribution; and a technique in which metal plasmon
effect or other light enhancement effect is additionally given to
produce much highly intensified light in order to record marks or
information pits smaller than the diffraction limit (for example,
techniques described in Patent Document 1, Patent Document 2,
Patent Document 3, Non-Patent Document 1, and Non-Patent Document
2).
[0009] An optical disc medium structure has been devised as another
conventional technique (for example, Non-Patent Document 4) to
further increase density in directions orthogonal (hereinafter,
referred to as "radial direction") to track extending directions:
information pits smaller than the diffraction limit are arranged as
a group track, and diffracted light due to a radially oriented
structure with periodically spaced group tracks of the small pits
and approximately flat portions therebetween is detected in radial
direction as a push-pull signal; thereby, controlling a
light-condensing spot, using tracking error signals with respect to
the group tracks obtained from the push-pull signal, to accurately
track each of pit trains arranged within an interval of the
diffraction limit and play back data of the desired small pits.
[0010] More specifically, a fundamental structure of the
super-resolution optical disc comprises a disc substrate that is
provided with small pits smaller than the diffraction limit and on
which formed is a film of light-absorbing material causing
super-resolution phenomena, such as
Silver-Indium-Antimony-Tellurium (AgInSbTe). On the disc, three
information-pit trains configured only with the small pits, form a
group track, which is regarded as a track having a width broader
than the diffraction limit. It has been reported (for example, in
Non-Patent Document 4) that a tracking error signal can be obtained
by detecting, as a push-pull signal, diffracted light due to the
structure described above, and that in order to scan the three pit
trains in the group track with a light-condensing spot, the
light-condensing spot is moved onto a desired pit train by adding
an electrical offset signal to the tracking error signal.
[0011] The conventional technique described above can record 1.5
times more densely than that without group tracks; however, in an
optical disc medium including pit trains with their width narrower
than the diffraction limit, it has been difficult to detect a
tracking error signal and take tracking-servo control while further
radially increasing its recording density on the optical disc
medium.
[0012] Patent Document 1: Japanese Patent Laid-Open No. 2004-87073
(page 9, FIG. 1)
[0013] Patent Document 2: Japanese Patent Laid-Open No. 2004-235259
(pages 4-5, FIG. 1)
[0014] Patent Document 3: Japanese Patent Laid-Open No. 2005-339795
(page 12, FIG. 2)
[0015] Non-Patent Document 1: J.J.A.P. Vol. 43, No. 7A, 2004, pp.
4212-4215, "Observation of Eye Pattern on Super-Resolution
Near-Field Structure Disk with write-Strategy Technique"
[0016] Non-Patent Document 2: J.J.A.P. Vol. 39, Part 1, No. 2B,
2000, pp. 980-981, "A Near-Field Recording and Readout Technology
Using a Metallic Probe in an Optical Disk"
[0017] Non-Patent Document 3: J.J.A.P. Vol. 45, No. 2B, 2006, pp.
1379-1382, "High-Speed Fabrication of Super-Resolution Near-Field
Structure Read-Only Memory Master Disc using PtOx Thermal
Decomposition Lithography"
[0018] Non-Patent Document 4: ODS Proceedings, WB4, pp. 203-205,
"Super-RENS ROM Disc with Narrow Track Pitch"
DISCLOSURE OF INVENTION
[0019] The present invention aims to solve a problem for increasing
density of the optical disc medium describe above, especially for
further increasing density in radial direction.
[0020] The present invention is to realize a density higher than
that obtained by conventional arts by providing information pits
arranged on approximately flat portions existing between group
tracks of the optical disc medium described at the background art,
and by detecting diffraction light due to the group tracks as a
push-pull signal, to obtain a tracking error signal.
[0021] For an optical disc device playing back data from the
optical disc medium according to the present invention, provided is
a tracking control method for enabling the device to read out pit
trains sequentially from an inner circumference to an outer one, or
from an outer circumference to an inter one.
[0022] The optical disc medium according to the present invention
includes information pit trains which are arranged spirally or
concentrically and whose depths are changed periodically at a pitch
in radial direction.
[0023] According to an optical disc medium and an optical disc
device of the present invention, accurate tracking operations are
realized to thereby play back data from or record data to
high-density information pits smaller than the diffraction limit,
which brings an effect that the optical disc medium can be played
back data sequentially from an inner circumference to an outer one,
or from an outer circumference to an inter one.
BRIEF DESCRIPTION OF DRAWINGS
[0024] FIG. 1 is an enlarged perspective view that illustrates an
information record surface of an optical disc medium of an
embodiment according to the present invention;
[0025] FIG. 2 is a cross-sectional view illustrating the structure
of the optical disc medium of the embodiment according to the
present invention;
[0026] FIG. 3 is a block diagram illustrating the configuration of
an optical disc device that plays back the optical disc medium of
the embodiment according to the present invention;
[0027] FIG. 4 shows an example of a simulation waveform with
respect to a tracking error signal obtained from the optical disc
medium of the embodiment according to the present invention;
[0028] FIG. 5 is a plane view illustrating a track format of the
optical disc medium of the embodiment according to the present
invention;
[0029] FIG. 6 is an enlarged view that illustrates a track
switching portion in FIG. 5 showing a track format of the optical
disc medium of the embodiment according to the present
invention;
[0030] FIG. 7 is a plane view illustrating another track format of
the optical disc medium of the embodiment according to the present
invention;
[0031] FIG. 8 is an enlarged view that illustrates a track
switching portion in FIG. 7 showing a track format of the optical
disc medium of the embodiment according to the present invention;
and
[0032] FIG. 9 is an enlarged view that illustrates a track
switching portion in a modified track format of the optical disc
medium of the embodiment according to the present invention.
REFERENCE NUMERALS
[0033] 1 disc substrate [0034] 2 recording layer [0035] 3
super-resolution film [0036] 30 optical disc [0037] 31 optical head
[0038] 32 system control unit [0039] 33 semiconductor laser [0040]
34 laser beam [0041] 35 collimation lens [0042] 36 beam splitter
[0043] 37 objective lens [0044] 38 optical sensor [0045] 39
actuator [0046] 40 summing amplifier [0047] 41 waveform shaping
unit [0048] 42 playback signal processing unit [0049] 43
differential amplifier [0050] 44 polarity controlling unit [0051]
45 tracking control unit [0052] 46 driving unit [0053] 47 address
processing unit [0054] 48 traverse controlling unit [0055] 49
record signal processing unit [0056] 50 laser driving unit [0057]
51 spindle motor [0058] 52 driving unit
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiment
[0059] An optical disc medium of the embodiment according to the
present invention will be explained with reference to the
figures.
[0060] FIG. 1 is an enlarged perspective view that illustrates an
information record surface of an optical disc medium of the
embodiment according to the present invention. A reference number 1
represents a disc substrate, a reference number 2 represents a
recording layer, and a reference number 3 represents a
super-resolution film for causing super-resolution phenomena. The
super-resolution film 3 has a nonlinear characteristic to light
intensity and is made of a light-absorbing material that changes
its refractive index or transmittance by light with an intensity
higher than or equal to a predetermined threshold value. As is
described, for example, in Patent Document 1 or Patent Document 2,
super-resolution phenomena occur by using, as the super-resolution
film 3, light-absorbing material composed of Sb (antimony), Te
(tellurium) or a compound including Sb and Te, or a metal oxide
such as ZnO, SnO.sub.2, TiO.sub.2, and Ta.sub.2O.sub.3 described in
Patent Document 3. As is described in Non-Patent Document 1 and
Non-Patent Document 2, typical material of them are AgInSbTe,
GeSbTe and the like. As for a material for the super-resolution
film 3, a nonlinear material causing surface plasmon phenomena or
other light enhancement phenomena may be used. By forming local
areas each of which sizes is smaller than or equal to the
diffraction limit at which light intensify is greater than or equal
to the predetermined value described above, a recording mark or
information pit (smaller than or equal to the diffraction limit)
that is provided on the optical disc medium can be played back.
[0061] Track groups T1, T2, and T3 each include information pit
trains P11, P12, and P13, information pit trains P21, P22, and P23,
and information pit trains P31, P32, and P33; then, the pit depth
of a track group differs from that of its neighboring one.
[0062] FIG. 2 is a cross-sectional view illustrating the structure
of the optical disc medium of the embodiment according to the
present invention. FIG. 2 shows that the track groups T1 and T3
have the same pit depth d1, and the depth d2 of the track group T2
is set as a value different from the depth d1 with a relationship
d1>d2.
[0063] For convenient sake, FIG. 1 shows only a portion cut out
from an optical disc medium of the embodiment according to the
present invention, illustrating that the track groups are
structured so that their pit depths d1 and d2 appear periodically
at a pitch in a direction indicated in the figure as radial
direction (X). In addition, FIG. 1 and FIG. 2 show a minimum
configuration for a lamination structure of the optical disc
medium; and the disc medium may further be overlaid with a cover
layer or a transparent layer to prevent chemical degradations and
mechanical damages.
[0064] Dash and dotted lines CT1, CT2, and CT3 drawn in FIG. 1 each
represent track centers of the track groups T1, T2, and T3, and the
distance between the track centers CT1 and CT3 corresponds to a
track group interval (a track group pitch).
[0065] Next, an optical disc device that plays back the optical
disc medium of the embodiment according to the present invention
will be explained. FIG. 3 is a block diagram illustrating a
configuration of the optical disc device. The figure shows only the
portions that concern the embodiment according to the present
invention, and other components may be added. In FIG. 3, a
reference number 30 represents an optical disc, and a reference
number 31 represents an optical head. A system control unit 32
includes a programmable-command-operation function so that the unit
controls the entire operations of the optical disc device. In the
optical head 31, there are a semiconductor laser 33, a laser beam
34 emitted from the semiconductor laser, a collimation lens 35 for
converting to a collimated light beam the laser beam 34 emitted
from the semiconductor laser 33, a beam splitter 36, and an
objective lens 37 for focusing on the optical disc 30 the laser
beam 34 passing through the beam splitter 36 to form a
light-condensing spot. An optical detector 38 receives light
returning from the optical disc 30; and in order that the
reflecting light is detected as a push-pull signal to obtain a
tracking error signal, the detector includes light receiving faces
that are divided into two faces by a dividing line in a direction
in which tracks (track groups, pit trains) of the optical disc 30
are extending. An actuator 39 has a driving structure that freely
drives the objective lens 37 in both directions of a direction
orthogonal to the track extending direction (hereinafter, referred
to as radial direction) and a direction perpendicular to an
information recording face of the optical disc 30, more
specifically, the optical axis direction of the objective lens 37
(hereinafter, referred to as a focusing direction).
[0066] A signal outputted from the optical detector 38 is inputted
into a summing amplifier 40; a waveform shaping unit 41 intensifies
modulated components of the outputted signal from an equalizer for
easily transforming the outputted signal from the summing amplifier
40 into a digital signal; and a playback signal processing unit 42
corrects errors of the digital signal outputted from the waveform
shaping unit 41 and demodulates the signal into playback data. Upon
receiving the output signals from the optical detector 38, a
differential amplifier 43 generates a tracking error signal as a
push-pull signal; a polarity controlling unit 44 reverses the
polarity of the tracking error signal on the basis of an output
signal S1 from the system control unit 32. On the basis of an
output signal S2 from the system control unit 32, a tracking
control unit 45 can add an electrical offset signal to a tracking
error signal processed by the polarity controlling unit 44. By
inputting into a driving unit 46 for the actuator 39 a tracking
error signal outputted from the tracking control unit 45,
tracking-position control of a light-condensing spot that the
objective lens 37 forms on the basis of the tracking error signal,
is taken in radially along the optical disc 30. An address
processing unit 47 extracts, from the playback data outputted from
the playback signal processing unit 42, position information about
the light-condensing spot on the optical disc 30, so as to send the
position information (that is, address information) to the system
control unit 32.
[0067] A traverse controlling unit 48 moves the optical head 31
radially along the optical disc 30 (that is, in radial direction)
on the basis of the tracking error signal outputted from the
tracking control unit 45 and an output signal S3 from the system
control unit 32 so that information pits can be accessed from an
inner circumference to an outer one. Record data is inputted into a
record signal processing unit 49, which produces and outputs an
emission pattern for the semiconductor laser 33 corresponding to
the data; a laser driving unit 50 feeds a current through the
semiconductor laser 33 in accordance with the emission pattern
outputted from the signal processing unit 49, to make the conductor
emit light.
[0068] When playing back the disc, a high-frequency current
superposed at the laser driving unit is fed to the semiconductor
laser 33 to make the laser emit light, resulting in reducing noise
of the semiconductor laser 33. Then, in order that the
super-resolution phenomena occur, as described above, the laser
driving unit 50 controls the quantity of the light emitted from the
semiconductor laser 33 so that the light intensity at the areas
smaller than or equal to the diffraction limit is increased to be
as high as or higher than a predetermined threshold value at which
refractive index or transmittance of the super-resolution film 3
changes. The record signal processing unit 49 and the laser driving
unit 50 are operated on the basis of output signals S41 and S42
outputted from the system control unit 32. When playing back the
disc, the laser driving unit 50 is controlled on the basis of the
output signal S41 so that the semiconductor laser 33 emits light
and stops emitting light. When recording, a signal based on the
output signal S42 is sent from the system control unit 32 to the
record signal processing unit 49 so that a signal corresponding to
data to be recorded is sent from the record signal processing unit
49 to the laser driving unit 50. The system control unit 32 also
sends the output signal S41 to the laser driving unit 50, so that
the semiconductor laser 33 can emit light modulated in accordance
with the data to be recorded. A spindle motor 51 rotates the
optical disc 30; a driving unit 52 controls rotation of the spindle
motor 51. In accordance with the output signal S5 from the system
control unit 32, the driving unit 52 changes rotation speed of the
motor and starts and stops rotating the motor.
[0069] FIG. 4A shows a simulation waveform of a tracking error
signal detected as a push-pull signal by the differential amplifier
43 in the optical disc device shown in FIG. 3 when the
light-condensing spot focused on the information recording surface
of the optical disc medium shown in FIG. 1 and FIG. 2 is displaced
radially along the optical disc medium (i.e., in the X
direction).
[0070] Simulation condition for the tracking error signal waveform
is that an application wavelength is .lamda.=405 nm, a numerical
aperture of the objective lens 37 is NA=0.85, and the small pit has
a rectangular shape and is smaller than or equal to a diffraction
limit (the diffraction limit is 0.119 .mu.m
(=.lamda./(4.times.NA)). In addition to the condition, the size of
the small pit is set as 0.075 .mu.m.times.0.075 .mu.m; and the
distance between the pit centers next to each other is set to 0.15
.lamda.m, and the track group pitch is set to 0.9 .mu.m.
[0071] Furthermore, a track group is set, similarly to the
structure shown in FIG. 1 and FIG. 2, as three pit trains having
its pit depths of d1=6.lamda./8 and d2=2.lamda./16. Here, the
values of the pit depths d1 and d2 are determined, for further
explanation from a viewpoint of light phases or phase differences,
under an assumption that a substantial refractive index n of the
light is one when the light passes through the transparent layer to
reach and focus on the information recording surface. However, in a
practical sense, the refractive index has a value equal to one or
larger; then, practical pit depths have been set at values obtained
by dividing said pit depths d1 and d2 by the refractive index n.
The same goes for a pit depth difference .DELTA.d explained
below.
[0072] In this simulation, the super-resolution film 3 is also
regarded as a film that has a constant refractive index (in the
simulation, it is assumed that the index is one), not varying with
light intensity. This is because that areas where the
super-resolution effects are produced are sufficiently smaller than
that of the light-condensing spot and varied components in
reflection light (so-called, RF signals) produced from the
super-resolution-effect areas are cancelled by differential signal
detection, which state negligibly contributes to the push-pull
signal treated as an operation signal in the simulation.
[0073] FIG. 4B is a simulation result, which shows amplitude
changes in the tracking error signal in relation to the pit depth
difference .DELTA.d (=d1-d2). In the simulation, the pit depth d2
is fixed as d2=2.lamda./16 and the pit depth d1 is changed. The
result shown in FIG. 4B reveals that the push-pull signal level
takes its maximum value when the pit depth difference .DELTA.d is
approximately 4.5.lamda./16, so that setting the difference as
approximately 3.lamda./16 through 6.lamda./16 can securely provide
a high-level tracking error signal.
[0074] In a waveform of the tacking error signal shown in FIG. 4A,
filled circles are servo-operation points for tracking control of
the pit trains P21, P22 and P23 having their pit depth d2 and
circles are points for tracking control of the pit trains P12, P13,
P31, P32 having their pit depth d1. Therefore, when tracking
control to a desired pit train is taken, the polarity controlling
unit 44 performs polarity setting that makes directions of the
servo operations coincide with inclination polarity of the tracking
error signal at the servo-operation point, and the tracking control
unit 45 performs adding operation of an electrical offset,
realizing excellent tracking operations at each servo-operation
point. These settings of the polarity and the offset are performed
according to the address information extracted by the address
processing unit 47.
[0075] As described above, even for discs having extremely narrow
track pitches, the tracking method can realize an excellent
tracking operation.
[0076] Next, a track format on the optical disc medium of an
embodiment according to the present invention will be
explained.
[0077] FIG. 5 is a top view of a track format on the optical disc
medium of an embodiment according to the present invention. A track
format of spiral type is shown in FIG. 5, in which a track group is
joined in a track switching portion 6a surrounded by a broken line
to the neighboring track group having a different depth.
[0078] FIG. 6 is an enlarged illustration of the track switching
portion 6a in FIG. 5. Square marks represent information pits 7.
Using FIG. 6, will be explained a control method that controls, in
relation to the track switching portion 6a, the tracking error
signal shown in FIG. 4.
[0079] In a track switching portion 6a, surrounded by the broken
line is a header area 8 which is provided for each pit train or
each track group and has a concave-convex structure. Because the
structure of the header area 8 is not an essential part of the
present invention, its detail explanation will be omitted on
purpose, provided that the structure enables the address processing
unit 47, when the area is scanned with a light-condensing spot, to
extract address information about where the light-condensing spot
is positioned on the disc. The system control unit 32 processes the
address information and then sends the next operation instruction
to the tracking control unit 45. An example is explained as
follows: after tracking the pit train P23 (a path I->a path II)
by servo operation, the servo operation point is changed to the pit
train P31 by, as described above, setting the polarity and the
electrical offset at the track switching portion 6a; thus, the
servo operation for tracking can be changed through a path III to
track the pit train P31. By repeating the sequence of operations
described above, data in all pit trains on the optical disc can be
played back sequentially from an inner circumference to an outer
one, or from the outer inner circumference to the inner one.
[0080] FIG. 7 is a top view of another track format for the optical
disc medium of the embodiment according to the present invention.
FIG. 8 is an enlarged illustration of the track switching portion
6b in FIG. 7. Numerals expressed as the same ones as those in FIG.
6 are the same components or equivalent ones, so that their
explanations will be omitted. Referring to FIG. 8, a method for
controlling tracking error signal shown in FIG. 4 will be explained
in relation to the track switching portion 6b. Similar to FIG. 6
described above, in a track switching portion 6b surrounded by the
broken line is a header area 8 which is provided for each pit train
or each track group and has a concave-convex structure. Because the
structure of the header area 8 is not an essential part of the
present invention, its detail explanation will be omitted on
purpose, provided that the structure enables the address processing
unit 47, when the area is scanned with a light-condensing spot, to
extract address information about where the light-condensing spot
is positioned on the disc. The system control unit 32 processes the
address information and then sends the next operation instruction
to the tracking control unit 45.
[0081] An example is as follows: after tacking the pit train P22 (a
path I->a path II) by servo operation, the servo operation point
is changed at the track switching portion 6b to the pit train P23
by, as described above, setting the polarity and the electrical
offset; thus, the servo operation for tracking can be changed
through a path III to track the pit train P23. By repeating the
sequence of operations described above, data in all pit trains on
the optical disc can be played back sequentially from an inner
circumference to an outer one, or from the outer inner
circumference to the inner one.
[0082] According to the track format shown in FIG. 8, the actuator
that holds and moves the objective lens 37 can substantially move
less, which brings a stable tracking servo operation.
[0083] FIG. 9 is an enlarged view of a track switching portion 6b
of a track format modified from that shown in FIG. 7, of the
optical disc medium of the embodiment according to the present
invention. Numerals expressed as the same ones as those in FIG. 6
and FIG. 8 represent the same components or equivalent ones, so
that their explanations will be omitted. Referring to FIG. 9, a
method for controlling tracking error signal shown in FIG. 4 will
be explained in relation to the track switching portion 6b. Similar
to FIG. 6 described above, in a track switching portion 6b
surrounded by the broken line is a header area 8 which is provided
for each pit train or each track group and has a concave-convex
structure. Because the structure of the header area 8 is not an
essential part of the present invention, its detail explanation
will be omitted on purpose, provided that the structure enables the
address processing unit 47, when the area is scanned with a
light-condensing spot, to extract address information about where
the light-condensing spot is positioned on the disc. The system
control unit 32 processes the address information and then sends
the next operation instruction to the tracking control unit 45.
[0084] An example is as follows: after tacking the pit train P23 (a
path I->a path II) by servo operation, the servo operation point
is changed at the track switching portion 6b to the pit train P31
by, as described above, setting the polarity and the electrical
offset; thus, the servo operation for tracking can be changed
through a path III to track the pit train P31. By repeating the
sequence of operations described above, data in all pit trains on
the optical disc can be played back sequentially from an inner
circumference to an outer one, or from the outer inner
circumference to the inner one.
[0085] According to the track format shown in FIG. 9, the actuator
that holds and moves the objective lens 37 substantially does not
need to move, which brings a tracking servo operation more stable
than that brought by the format shown in FIG. 8.
[0086] In the embodiments according to the present invention, the
shape of the pit has been a rectangle, as shown in FIG. 1 and FIG.
2; however, the shape is not limited to that shape, it may be a
circle, an ellipse or the like.
[0087] In FIG. 1 and FIG. 2, showing the embodiment according to
the present invention, all information pits 7 have been equally
spaced to be arranged, however, when information data are actually
recorded, all of the pits are not necessarily needed; the
information pits 7 may be randomly spaced in an array direction to
be arranged, the array-direction lengths of the information pits 7
are not necessarily constant, and the lengths may vary according to
record data and modulation methods.
[0088] When the array-direction length of any information pit 7 is
made equal to or shorter than the diffraction limit, it is
advantageous that interference from diffraction light supposed to
occur by pits with their lengths equal to or longer than the
diffraction limit can be avoided accordingly. If there is an
optical system to be applied which can dissolve the above
interference problem, it is obviously needless that the
array-direction length of any information pit be made equal to or
shorter than the diffraction limit.
[0089] Although the above explanation is made with the number of
pit trains in a track group being three, the pit train number is
not limited to thereto and it may be various number provided that
each pit train has radially a width equal to or narrower than the
diffraction limit and the pitch (or interval space) between the
track groups is equal to or wider than the diffraction limit.
[0090] When the pitch (or interval space) between the track groups
is made approximately 1.6 .mu.m, approximately 0.74 .mu.m, or
approximately 0.34 .mu.m, it becomes the same as the track pitch as
that of existing CD, DVD, or BD, respectively, which can enhance
compatibility on servo operations such as track jump operations, in
optical disc systems.
[0091] In the embodiments according to the present invention, it
has been described that each pit train includes pits whose shape
are concave; however, the pit does not necessarily have a concave
shape, it may be a "protrusion" in a convex shape. In this case,
the pit depth difference .DELTA.d (=d1-d2) can be replaced with the
height difference between the "protrusions". Furthermore, a track
group has the "protrusions" in a convex shape but its neighboring
track group may have the pits whose shapes are concave. In this
case, the pit depth difference .DELTA.d (=d1-d2) is calculated as
d1>0 and d2<0. In each of the cases, the concave-convex
portions produce phase differences, which brings the same effects
as the embodiments.
* * * * *